1. Field of the Invention
The present invention relates to a method for forming a silicon nitride film having low leakage current and a high break down voltage, more specifically to a method for forming a ultra thin silicon nitride capacitor film having low-leakage current characteristics and a high break down voltage.
2. Description of Related Art
Recently, semiconductor devices have been highly integrated and have become finer, so that capacitor dielectric films of dynamic-random access memories (DRAMs) have become thinner. For example, silicon nitride films which have come in practical use as the capacitor dielectric films have thickness of equal to or less than 10 nanometers.
The silicon nitride capacitor film has been formed by low pressure chemical vapor deposition (LPCVD) using a mixture of a gas of the silane family, for example, dichlorosilane (SiH.sub.2 Cl.sub.2) gas and ammonia (NH.sub.3) vapor, in which mixture rate of SiH.sub.2 Cl.sub.2 : NH.sub.3 is 1:5 to 1: 10, at a temperature of 680.degree. to 800.degree. C. (H. Tanaka et al., IEEE/IRPS 31 (1992)). In this process, due to a native oxide layer on a surface of a silicon substrate or a lower electrode of poly-silicon and air ingress into a LPCVD apparatus during loading of silicon wafers in the LPCVD apparatus, oxygen enters into the silicon nitride film so that defects at which stoichiometric composition (Si: N=3: 4; Si.sub.3 N.sub.4) is broken are created.
The defects, which are caused by broken or distorted bonds between atoms and so called "weak spots" are liable to be starting points of dielectric break down, which degrades a dielectric strength of the silicon nitride film. In addition, portions at which the bonds between atoms are broken are prone to be oxidized which induces further ingress of oxygen into the silicon nitride film so that the silicon nitride film is degraded and its dielectric constant is decreased.
In a prior art, in order to improve reliability of the silicon nitride film, there is provided a process including steps of removing a native oxide layer and immediately forming a thin silicon nitride film having a stoichiometric composition on a capacitor electrode by rapid thermal nitridation (RTN) in ammonia vapor prior to LPCVD so as to prevent growth of uncontrollable oxide layer which are undesirably on the capacitor electrode formed by only LPCVD (K. Ando et al., Appl. Phys. Lett. vol. 59, No. 9 pp. 1081-1083 (1991)).
Referring to FIGS. 1A to 1E, the above conventional process for forming a silicon nitride film will be described. FIGS. 1A to 1E are schematic sectional views illustrating the above conventional process.
As shown in FIG. 1A, field oxide layers 102 having a thickness of several thousands angstroms are formed on a silicon substrate 101 under oxygen atmosphere so as to separate a device region. Then, a poly-silicon film 103 having a thickness of 1000-2000 angstroms is formed as a lower electrode of a stacking capacitor on the silicon substrate 101 by CVD, as shown in FIG. 1B.
Thereafter, a native oxide film on the poly-silicon film 103 is removed so as to expose a surface of the poly-silicon film 103 by a processing using weak hydrofluoric acid etc. Immediately, the surface of the poly-silicon film 103 is nitrized by a rapid thermal process having a duration of 60 seconds in ammonia vapor under atmospheric pressure at a temperature of 800.degree. to 1000.degree. C. (RTN) so as to form a thin silicon nitride film 104 having a thickness of 10 to 20 angstroms, as shown in FIG. 1C.
Then, a nitride film 105 having a thickness of 50 to 100 angstroms is formed on the thin silicon nitride film 104 by LPCVD with a mixture of a gas of the silane family such as monosilane gas or dichlorosilane gas and ammonia at a temperature of 700.degree. to 800.degree. C., as shown in FIG. 1D. A poly-silicon film 106 is formed as an upper electrode on the capacitor silicon nitride film 105 by CVD so that a capacitor of DRAM is completed, as shown FIG. 1D.
In the conventional LPCVD, the silicon nitride film is deposited by using only one of silane gases and ammonia vapor, so that Si-N bonds are formed by reactions of species of silylenes such as SiCl.sub.2 and SiH.sub.2 with ammonia. However, it is known that the species of silylenes are also easy to form Si--Si bonds. Therefore, in the above mentioned process, though a stoichiometric thin silicon nitride film exists only on the poly-silicon film of the lower capacitor electrode, which is formed by the RTN process, the capacitor silicon nitride film includes many Si--Si bonds since the most of the silicon nitride film is formed by the conventional LPCVD. The Si--Si bonds in the silicon nitride film are conductive so that the leakage current increases as the Si--Si bonds increase. In addition, the structure of the silicon nitride film is considered to be distorted near the portions of the Si--Si bonds, which becomes the weak spots. Therefore, dielectric break down is prone to occur by increase in the Si--Si bonds.
In order to prevent the Si--Si bonds formation and to form a stoichiometric silicon nitride film, the flow rate of the ammonia gas is increased relatively to the silane gas in the conventional LPCVD process. However, by the above conventional process, it is impossible to selectively suppress the formation of the Si--Si bonds so that the problem of the formation of the Si--Si bonds can not be fundamentally resolved. Consequently, it is difficult to form a bulk stoichiometric silicon nitride film using the RTN+LPCVD process as shown in FIGS. 1A to 1E.
In another prior art, there is provided a process in which an reliable oxide film is formed on a silicon wafer by CVD using a mixture of gases including hydrogen chloride (HCl), as shown Japanese Patent Application Laid-open No. 156637/1990. In this process, silicon wafers are loaded into a reactive furnace of, for example, a quartz pipe and the reactive furnace is evacuated to a pressure of on the order of 1.0 Torr. Thereafter, nitrogen (N.sub.2) is introduced into the reactive furnace so as to replace inner atmosphere with nitrogen atmosphere. Then, the inside of the reactive furnace is heated to a temperature of 600.degree. to 900.degree. C. Mixture gas including silane (SiH.sub.4) or disilane (Si.sub.2 H.sub.6) having a flow rate of 30 cm.sup.3 /minute, dinitrogen monoxide (N.sub.2 O) having a flow rate of 1.5 cm.sup.3 /minute and hydrogen chloride having a flow rate of 1 to 1000 cm.sup.3 /minute is introduced so that oxide films are grown on the silicon wafers. In this process, the hydrogen chloride getters impurity metals at temperatures of higher than 400.degree. C., so that impurity metals in the oxide films are removed so as to prevent initial shortage of break down voltage and to reduce leakage current.
If the above process is applied to the formation of the silicon nitride film using ammonia instead of dinitrogen monoxide, thermal decomposition of dichlorosilane is suppressed since hydrogen chloride is formed by the decomposition. Therefore, formation of reactive species (SiCl.sub.2, SiHCl) which are also formed by the decomposition and contribute the deposition of the silicon nitride film are prevented, so that a higher process temperature is necessary to obtain a practical deposition rate. However, the higher process temperature promotes the formation of the Si--Si bonds so that a composition of the silicon nitride film greatly deviates from the stoichiometric composition.
In addition, hydrogen chloride is prone to join with ammonia so as to form ammonium chloride (NH.sub.4 Cl) in a vapor phase, which contributes particles and which prevents hydrogen chloride from being absorbed on a surface of the depositing silicon nitride film. Therefore, in order to improve the stoichiometry of the silicon nitride film by absorbing hydrogen chloride on the surface of the silicon nitride film, it is considered to be insufficient to add hydrogen chloride to the growing gas in the LPCVD process.